The Earth rotates around its axis and also revolves around the Sun. In both these motions, centripetal acceleration is present. Therefore, strictly speaking the Earth or any frame of reference fixed on Earth can not be taken as an inertial frame. However, as we are dealing with speed ≈× 108m/s, (speed of light) and speed of Earth is only about 3 × 104 m/s, therefore when small time intervals are involved, effect of rotation and revolution of Earth can be ignored. Further more, this speed of the Earth can be assumed to be constant. Hence, the Earth or any other frame of reference set up on the Earth can be taken as approximately inertial frame of reference.
On the contrary, a frame of reference which is accelerated is non-inertial frame.
Other examples of inertial frames of reference are :
- A frame of reference remaining fixed w.r.t. Stars.
- A spaceship moving in outer space, without spinning and with its engine cut off.